"It's a growing problem and I think it's going to get worse," said Larry Roberts of McMaster University in Canada.

"My personal view is that there is a major public health challenge coming down the road in terms of difficulties with hearing," said Roberts.

The researchers interviewed and performed detailed hearing tests on a group of 170 students between 11 and 17 years old, learning that almost all of them engage in "risky listening habits" - at parties, clubs and on personal listening devices - and that more than a quarter of them are already experiencing persistent tinnitus, a ringing or buzzing in the ears that more typically affects people over 50.

Further testing of the same subjects showed that even though they could still hear as well as their peers, those experiencing tinnitus were more likely to have a significantly reduced tolerance for loud noise, a sign of hidden damage to the nerves that are used in processing sound, that can foretell serious hearing impairment later in life.

Roberts said that when the auditory nerves are damaged, brain cells increase their sensitivity to their remaining inputs, which can make ordinary sounds seem louder.

Increased loudness perception is an indication of nerve injury that cannot be detected by the audiogram, the standard clinical test for hearing ability.

Neuroscience research indicates that such "hidden hearing loss" caused by exposure to loud sounds in the early years deepens over the life span, worsening one's hearing ability later in life, researchers said.

"The levels of sound exposure that are quite commonplace in our environment, particularly among youth, appear to be sufficient to produce hidden cochlear injuries.

The message is, 'Protect your ears'," said Roberts.

It is common after listening to loud music to experience a ringing in the ears for the next day or so, said Roberts, who collaborated with researchers at the University of Sao Paulo School of Medicine for the study.

More than half the students in the study said it had happened to them.

This brief tinnitus is an early warning sign of vulnerability to the injurious effects of noise exposure, according to Roberts. Testing showed that 28 percent of the study participants had already developed persistent tinnitus.

The 28 percent of participants with persistent tinnitus also showed heightened sensitivity to loud sounds, indicating that the neurons that transmit sounds to the brain may have been damaged, said Roberts.